One of the reasons why Nintendo does so well is that they ship (mostly) completely finished products with their games.
On the whole the only people who care about that are hardcore collectors and serious gamers, but they are the minority of all gamers though. That only matters to a tiny portion gamers but it seems like a lot more to you because you travel in those circles and see it discussed a lot. But most people who play games do not care about it.
Nintendo does well because they are a friendly and non threatening company in general. They don't get involved in scandals, all of their first party games are safe and soft for all ages so parents have no issue blindly buying them for kids, their spokesmen are always friendly and soft speaking people with big warm smiles. Basically they have a public image on one can be upset with or be turned off by.
Then you have the nostalgia factor as Nintendo was a huge staple in gaming's old days when it really took off. Nintendo was there with Mario, kirby, pokemon, Mario kart, metroid, zelda, etc. That's why they continue to do not much besides milk the same old franchises over and over and over again. They just stick with what's safe and well known. Anytime nintendo does create a new game like arms or Wii music it's quickly forgotten in favor of yet more zelda and Mario character's.
But their biggest success is merchandising. They merchandise the living shit out of their brands. Board games, stickers, trading cards, clothing, diapers, nintendo monoply, mario spaghettios, kites, amigos, figures, toys, bikes, game accessories, posters, blankets, dishes, kid's chairs, and a thousand other items. Without it they would be dead in the water.
Nintendos success is built on not taking risks, not creating new things, repetition, being as non threatening as possible and by milking well known franchises.